WASHINGTON (AP) — The White House released on Thursday the following letter on Syria that was sent to Sens. John McCain, R-Ariz., and Carl Levin, D-Mich., by Miguel E. Rodriguez, assistant to the president and director of the Office of Legislative Affairs:

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Thank you for your letter of April 24 regarding the situation in Syria and the allegations of chemical weapons use there. I am responding on behalf of the president and want to offer a prompt response to your question: “Has the Assad regime— or Syrian elements associated with, or supported by, the Assad regime— used chemical weapons in Syria since the current conflict began in March 2011?”

At the president’s direction, the United States government has been closely monitoring the potential use of chemical weapons within Syria. We have kept the relevant committees of Congress fully informed of our assessments on this issue, consistent with our statutory obligations. Our intelligence community does assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin. This assessment is based in part on physiological samples. Our standard of evidence must build on these intelligence assessments as we seek to establish credible and corroborated facts. For example, the chain of custody is not clear, so we cannot confirm how the exposure occurred and under what conditions. We do believe that any use of chemical weapons in Syria would very likely have originated with the Assad regime. Thus far, we believe that the Assad regime maintains custody of these weapons and has demonstrated a willingness to escalate its horrific use of violence against the Syrian people.

Because of our concern about the deteriorating situation in Syria, the president has made it clear that the use of chemical weapons— or transfer of chemical weapons to terrorist groups — is a red line for the United States of America. The Obama administration has communicated that message publicly and privately to governments around the world, including the Assad regime. We have also provided information and equipment to the region to help protect Syrians and support humanitarian workers in their life-saving work. However, precisely because the president takes this issue so seriously, we have an obligation to fully investigate any evidence of chemical weapons use within Syria. That is why we are currently pressing for a comprehensive United Nations investigation that can credibly evaluate the evidence and establish what took place. We are also working with our friends and allies, and the Syrian opposition, to procure, share and evaluate additional information associated with reports of the use of chemical weapons so that we can establish the facts.

Given the stakes involved, and what we have learned from our own recent experience, intelligence assessments alone are not sufficient— only credible and corroborated facts that provide us with some degree of certainty will guide our decision-making and strengthen our leadership of the international community. The Obama administration will remain in close consultation with you and the Congress on these matters. In the interim, the administration is prepared for all contingencies so that we can respond appropriately to any confirmed use of chemical weapons, consistent with our national interests. The United States and the international community have a number of potential responses available, and no option is off the table. In response to the deteriorating situation in Syria, we have also dramatically increased our humanitarian assistance and our support for the opposition to bring about the political transition that the Syrian people deserve.